Celestial Mechanics

NASA engineers take perverse pride in pointing out that there is hardly a single aspect of the space station that relies on new technology. Of course, there’s the free-flying basketball-size flashlight and video camera that will help astronauts see what is going on during space walks (extravehicular activities, or evas), and the new backpack propulsion units they’ll wear (to be used only in emergencies). But almost everything else, from the solar arrays that provide the power, to the pressurized living quarters, to the astronauts’ hand tools, has been tried and tested in one form or another on past missions. In most respects it will be an engineer’s dream.

There is, of course, one obvious exception, which NASA engineers, despite their outward calm, have not been able to overlook: the space station will be very, very large. Too large, in fact, to be launched fully assembled from the ground.

To the average ground-based person, this is a fact pregnant with meaninglessness. Why not just launch the thing in parts and assemble it in space? To an engineer, who never wants to leave anything to chance, the prospect is terrifying. It introduces a whole new level at which things can go wrong. Assembly, all of a sudden, is not something you do in the well- controlled environment of a hangar, with teams of engineers waiting on call in case a problem arises.

The space station is being built over five years, and there’s no way you could wait to put it all together, test it, and break it apart and fly it up in chunks, says Wilbur Trafton, NASA’s flight director for the project. NASA engineers thus resorted to the time-honored practice of dividing the problem into parts. The first part--how to assemble the behemoth--was solved fairly easily from an engineering standpoint but is potentially dangerous to the astronauts who have to carry it out. The other part--how to make sure it all works--is the engineering equivalent of a giant leap.

NASA engineers essentially had two choices for how to assemble the station. They could have built each component in such a way that fitting them together could have been done with no human intervention whatsoever. This option would have required making each connection foolproof and equipping it with bulky motors, which would have sent costs, already high, impossibly higher. Instead NASA has by necessity opted for a bare-bones design that relies on labor-intensive assembly to an unprecedented degree. U.S. and Russian astronauts are expected to spend at least 800 hours in space assembling the station, twice the total hours of eva work U.S. astronauts have logged to date.

The space station will have a long central truss for its backbone, which will support several large arrays of solar cells and other instruments. Rocket engines will give the station a boost every once in a while so that trace molecules of atmosphere don’t succeed in slowing it down and forcing it to plunge Earthward. (This is the job of Russia’s service module, which has been delayed at least six months from its original launch date of April 1998.) The various pressurized modules built by different participating countries will attach to the truss and to each other, forming one long snaking pressurized passageway. To be fair, not every bolt will have to be tightened by human hands. The Canadian robotic arm will be available to do the really heavy lifting. NASA has also designed some connections between the station’s prefab modules that can hook up automatically. For instance, the beams that make up the central truss--to which the service modules, solar arrays, and other parts will attach--have joints with cone-shaped receptors and motorized mechanisms. But for each new module that is added to the truss, astronauts will have to venture out into the vacuum of space to connect by hand dozens of electrical wires that conduct power and data, and hoses that carry water and other fluids.

These repetitive, mindless chores are the kinds of jobs astronauts dread because their very triviality seems to invite errors. Despite the glamour, working in space is actually a lot like working on your car: there is no reason everything shouldn’t always go well, but in practice it rarely does. Although all the connectors have been designed to accommodate balloon-gloved astronauts, the extreme cold of space has a way of stiffening cables and causing connectors to stick. Going outside is the biggest bite of hazard, says astronaut Jim Newman. You’re your own little spaceship, and there’s very little material separating you from space. And there’s always something that’s going to go wrong. The trick is, what’s it going to be?

Newman should know. In a practice flight in 1993, he spent six straight hours gripping wrenches and attaching tethers, bolting and unbolting subsystems, all the time wearing pressurized gloves that encased each finger. With relief and aching hands, he stowed his tools away into the space shuttle’s cargo bay and looked forward to taking his space suit off and kicking his feet up in zero-g. Then the door to the toolbox jammed open. Newman tugged at it, banged with his hand, pried it with a crowbar. He couldn’t get much leverage, because each action tended to send him floating away from the toolbox in reaction. It took the better part of an hour for Newman and a colleague to persuade the door to close. That’s just the kind of thing you don’t need at the end of a long eva, he says. Unfortunately, it’s also the kind of thing the astronauts charged with assembling the station will have to get used to.

Figuring out how to make sure the station works once you’ve put it together is a more subtle problem. For a normal-size project--say, the space shuttle--it’s fairly routine. First you build the parts and test them, and when you’ve assembled them you test the whole shebang. For the space station, this last step can be done only in orbit. Compounding the problem, some parts will be launched before others are even built. Consider the situation from the standpoint of the poor engineers on the ground. You build part A, which has to connect to part B. But as soon as you finish part A, you have to pack it off into a rocket and send it into orbit. By the time you finish part B, how do you know it will really work when you connect it to part A?

NASA and its contractors decided to do the next best thing: simulate each and every part on computers. That way, even if part A is 250 miles up, at least you can refer to something. Of course, it’s not as easy as all that. The drawback is that no matter how clever you are about designing your computer simulation, it’s no substitute for having the real hardware. To make a simulation, you have to start by making assumptions about how your hardware will behave in certain situations--how much it will expand when the sun heats it, how much drag stray molecules from Earth’s atmosphere will exert on its surfaces, and so on. There are lots of data to draw on, but they all tend to involve average behaviors rather than actual ones. This is why engineers like to put the wing of an airplane, for instance, in a wind tunnel even though they’ve already used computer models to simulate how air should flow over it. They want a reality check.

The solution is to take the traditional distinction between the phases of manufacturing, test, and deployment and blur them. The space station’s wind tunnel will be its actual flight. Once part A is orbital, astronauts will busy themselves taking measurements of all kinds and relaying the data back to engineers on the ground. These data, which describe the true behavior of the hardware, will be incorporated into the simulations to make them more accurate. Once we get stuff in orbit, we’ll be able to measure the performance in the space environment and see how that corresponds to what we predicted in the model, and make adjustments for it, says Mike Hawes, a senior engineer on the space station project. Such a practice will also give the engineers more flexibility to respond to such contingencies as the Russians’ postponement of the service module launch.

Using simulations in this way is not entirely without precedent. Boeing, the prime U.S. contractor for the station, pioneered the approach in making its 777 passenger plane, which is so complex that Boeing wanted accurate simulations to supplement field trials. It is a practice that engineers will rely on increasingly over the coming decades for complex, difficult-to-test products, from airplanes to far-flung computer networks and perhaps even automobiles. But it will be a few years yet before engineers are entirely comfortable not being able to kick the tires.